Recently, with the generalization of information communication services, the introduction of various multimedia services, and the advent of high-quality services, a demand on radio communication services has been rapidly increased. To satisfy such a demand, various radio communication techniques are being researched in many fields. Examples thereof include various diversity schemes and a cooperative transmission scheme using neighboring base stations.
The diversity scheme implies that the same data is transmitted in a repetitive and/or redundant manner to ensure reliability of communication. When the same data is transmitted through a plurality of independent paths, i.e., a diversity branch, even if an error occurs in data of a certain path, original data can be recovered from data of other remaining paths. Therefore, the diversity scheme is a method proposed to achieve reliable data transmission and/or reception through independent multiple paths. Examples of the diversity scheme include a frequency diversity scheme of transmitting signals at different frequencies, a time diversity scheme of transmitting signals at different times, a spatial diversity scheme of using a plurality of transmit (Tx) antennas, etc.
Research on a multi-cell cooperative radio communication network capable of obtaining a diversity gain by using cooperative communication of a plurality of base stations has actively been conducted in recent years. The multi-cell cooperative communication has been introduced to provide cell coverage extension, throughput enhancement, performance enhancement in a cell edge region, etc. Such a ‘multi-cell cooperative radio communication system’ can be regarded as a radio communication system performing diversity transmission by using a plurality of relay stations.
Meanwhile, in a global system for mobile communication (GSM)/general packet radio service (GPRS)/enhanced data rates for GSM evolution (EDGE) communication system or a 3rd generation or next generation radio communication system, a mobile station periodically measures a receive (Rx) signal level and signal quality of a serving cell, i.e., a cell to which the mobile station belongs. Information on the measured Rx signal level and/or signal quality is used for various purposes, and in particular, can be used to determine power that is output from the mobile station for uplink (UL) transmission (hereinafter, referred to as ‘UL power’).
Controlling of UL power is a basic element of a radio communication system. The purpose of controlling UL power is to regulate a magnitude of a signal received from a base station to an appropriate level. By maintaining the magnitude of the received signal to the appropriate level, unnecessary power consumption of the mobile station can be avoided, and a data transfer rate or the like can be adaptively determined, which is effective to improve transfer efficiency.
In general, controlling of UL power is classified into two elements, i.e., open-loop power control and closed-loop power control. The former includes an operation of compensating for UL Tx power by measuring (or estimating) a path-loss for a downlink (DL) signal and then by predicting a path-loss for a UL signal and an operation of determining UL power by considering an amount of radio resources allocated to a corresponding mobile station or an attribute of data to be transmitted. The latter is an operation of controlling UL power by using information included in a closed-loop power control message or the like delivered from the base station. Equation 1 shows a method of determining UL Tx power as described above.P(i)={PMAX, a×PL+A(i)+f(i)} [dBm]  [Equation 1]
Herein, P(i) denotes UL Tx power at an ith time point, and PMAX denotes maximum Tx power of the mobile station. In addition, PL denotes an estimated path-loss value for a DL signal. α and A(i) denote an attribute of data to be transmitted and a high-layer signal at the ith time point and a parameter given by an amount of allocated resources, and these two components correspond to open-loop power control. f(i) denotes a power control value at the ith time point determined by information included in a closed-loop power control message provided from the base station, and corresponds to closed-loop power control.
In Equation 1, a primary purpose of the open-loop power control is to regulate a magnitude of a Tx signal provided from a mobile station, i.e., UL power, to a appropriate level by considering an estimated or calculated path-loss level for a DL signal under the assumption that a path-loss level for a UL signal coincides with a path-loss level for a DL signal. Herein, an appropriate magnitude of the Tx signal is determined by the parameter A(i). The purpose of the closed-loop power control corresponding to f(i) of Equation 1 is to compensate for inconsistency of path-losses for UL and DL signals and channel fading that changes with a faster time-scale than an average signal path-loss.
As described above, the conventional method of controlling uplink (UL) power includes an open-loop power control parameter and a closed-loop power control parameter. Among them, the former is a parameter for performing power control by estimating a path-loss for a downlink (DL) signal from a base station (BS) in a cell to which a mobile station (MS) belongs and by compensating for the path-loss for the DL signal. For example, if a distance from the MS to the BS to which the MS is connected is increased and thus the path-loss for the DL signal is great, UL power is controlled to increase UL transmit (Tx) power. The latter is a parameter for controlling UL power in such a manner that the BS directly delivers information (i.e., a control signal) required for regulation of UL Tx power.
However, cooperative communication between BSs is not considered in the conventional method of controlling UL power. More specifically, according to the conventional method of controlling UL power, it is assumed that, in open-loop power control, each MS considers only a path-loss for a signal from a BS in a cell to which the MS belongs, and in closed-loop power control, each MS receives a control signal also from the BS in the cell to which the MS belongs. Therefore, direct use of the aforementioned conventional method of controlling UL power is inappropriate in an environment where a UL signal is transmitted in cooperation of several BSs.
For example, assume that a UL signal with sufficient quality can be created in cooperation with neighboring BSs even if a path-loss for a signal from the BS in the cell to which the MS belongs is great. In this case, UL power has to be increased to some extent if the conventional method is directly used (i.e., if cooperative communication between BSs is not considered). The increase in the UL power of the MS results in the increase in energy consumption of the MS. In addition, a signal transmitted with high power may have an adverse effect on communication of other MSs.